The subject invention refers to a cylinder for a two-stroke, crankcase scavenged internal combustion engine, comprising an inlet pipe for air/fuel mixture, at least one transfer port, and at least one outer connecting port intended to be connected to a connecting duct, whereby the outer connecting port is arranged so that it can be connected to the mentioned transfer port via a piston ported air passage.
Thus, fresh air is added at the upper part of the scavenging ducts and is intended to serve as a buffer against the air/fuel mixture below. When the scavenging takes place, this air buffer is largely lost out into the exhaust outlet, thus reducing the fuel consumption and exhaust emissions.
Engines of the above mentioned type have ducts for fresh air designed with connecting ports in the cylinder, usually one on each side of the piston. The air flow between these connecting ports and the scavenging ducts is controlled by the position of the piston by means of a recess in the piston, which in a defined piston position connects the connecting ports and the scavenging ducts"" scavenging ports in the cylinder. In order for this piston control to function, it is necessary that the connecting ports are positioned at a defined distance above the inlet port.
According to known technology, which for instance is illustrated in WO98/57053, cylinders in engines of this type have therefore been designed with one or several air ducts positioned above the inlet pipe. However, this positioning, which is closer to the combustion chamber then the inlet pipe, entails that the air inlet in this case is subjected to a relatively high temperature with air expansion and reduced delivery rate as a result. Furthermore, the air inlet disturbs the cooling air, which usually can flow unimpeded around the cylinder above the inlet tube, with an even higher temperature as a result.
In accordance with another known variant, which is also illustrated in WO98/57053, the air inlet is designed with two air ducts, one on each side of the inlet. In this case, the inlet has been located higher than usual, and to prevent the inlet being blocked by the piston, an aperture has been made in the piston, which allows a passage for the air/fuel mixture down into the crankcase, even when the piston""s lower edge is positioned below the inlet. Consequently, this solution requires a considerably more complicated piston design.
The purpose of the subject invention is to create a cylinder to which it is possible to add fresh air to the scavenging ducts without causing the problem mentioned above.
This purpose is achieved according to the subject invention by means of a cylinder of the type mentioned initially, where an outer connecting port is located below the inlet tube and that at least one passage extends from the outer connecting port, obliquely upwards through the cylinder wall, to at least one connecting port.
In relation to the inlet pipe, the mentioned connecting port is located so that a satisfactory control of the air supply by the piston to the scavenging ducts is possible.
This design enables the cooling air to flow freely around the cylinder above the inlet tube, which improves the cooling of the cylinder. Furthermore, the fresh air is not subjected to the higher temperatures which prevail above the inlet pipe, since the wall passages pass from below and past the inlet pipe through the material of the cylinder wall.
Further advantageous cooling of the fresh air is obtained by means of the close contact with the inlet, where each passage winds itself around the inlet pipe and is consequently cooled as a result of this.
In accordance with a preferred embodiment, the cylinder comprises two wall passages, which pass one on each side of the inlet pipe.
The two wall passages can be joined in a common outer connecting port, which in that case is located immediately below the inlet tube. This embodiment can be achieved with a minimal amount of material resulting in a light and inexpensive cylinder.
Each scavenging port is preferably located slightly above the corresponding connecting port, which entails that the fresh air at the passage between the connecting ports and the scavenging ports is made to flow obliquely upwards. The location of the air inlet, according to the subject invention, therefore gives the fresh air an advantageous flow direction for the piston ported air passage, i.e. obliquely upwards, whereby unnecessary directional changes of the fresh air flow is eliminated.
The inlet pipe is preferably directed obliquely downwards towards the cylinder. The wall passages, which are directed obliquely upwards and which pass the inlet tube, thereby pass the inlet pipe at a greater angle, preferably almost at a right angle, which results in an even further reduction of material consumption. Furthermore, the obliquely downwards directed inlet pipe is favourable, since the air/fuel mixture taken in is made to flow down into the crankcase and consequently improve the lubrication of the big end bearing.